The unofficial holiday was created in the early 1960s by Princeton mathematics students, David Kelly and Michael Spivak, while working on the special properties of the number 17.

It is unclear why the day was called Yellow Pig Day or what a yellow pig has to do with the number 17, though legend has it that it is in reference to David Kelly's collection of yellow pigs. The mascot of the holiday, the yellow pig, has 17 toes, 17 eyelashes, and 17 teeth.

A Number With Special Properties

17 is a prime number that has significance in mathematics. It is the sum of the first four prime numbers - 2, 3, 5, and 7. A prime number is a number that can only be divided by 1 and by itself. Several studies have shown that when people are asked to choose a number between 1 and 20, a majority of people choose the number 17.

17 is not always a special number. In Italy, for example, 17 is considered to an unlucky number.